Switching device for echo blocking and feed-back blocking



Jan. 28, 1941. E, HGLZ ER 2,229,806

SWITCHING DEVICE FOR ECHO BLOCKING AND FEED-BACK BLOCKING Filed Aug. 51, 1938- CONTROL 2 CONT 0 DE V/CE f;

ENTOR. ER wnv HgQ ZLER A TTORNEY.

DETECTOR Patented Jan. 28, 1941 UNITED STATES SWITCHING DEVICE FOR ECHO BLOCKING t AND FEED-BACK BLOCKING Erwin Htilzler, Berlin-Halensee, Germany, as signor to Fides Gesellsch'aft fiir die Verwalt-ung und Verwertung Von gewerblichen Schutzrechten mit beschrinkter Haftung, Ber.- lin, Germany, a corporation of Germany Application August 31, 1938, Serial No. 227,648

In Germany September 7, I937 Claims.

In the transmission of speech currents there often exists the problem of providing a control action depending upon two voltages derived from the transmission paths of the speech current.

,. Such control actions not only are required in case of echo blocking means and feed back blocking means but also for the switching of amplifiers to two difierent directions of communication. A control arrangement responding to two voltages is 0 also to be provided in the operation of call receivers that are to respond in accordance with the diflerent directions of transmission in use. It is hereby required that at the action of the one voltage a control action is brought about in a definite direction and at action of the other voltage this control action is to take place in the opposite direction. If the two voltages disappear following the performance of a definite control action, the operating state last switched to is to be retained.

If both voltages act at the same time, it is desired that either the respectively higher voltage produces the actual control or that if when a voltage is present and at the state brought about by this voltage, another voltage is added, a reverse switching takes place as soon as the added voltage has attained a certain predetermined limit.

In accordance with the invention these actions are obtained by means of a control circuit ary I rangement consisting of a relaxation tube relay having two separate inputs and to which the two voltages are applied separately. This circuit consisting in its simplest form of two tubes biased in the lower bend and whose plate circuits are coupled respectively with the grid circuit of the other tube, was found to be especially useful in echo blocking means and feed-back blocking means and can also be used equally well for the other purposes mentioned above. The blocking means provided by my invention is adapted to be con- 44) trolled by a small difierence of potential of a signalling voltage in one direction in excess of the existing signalling voltage in the opposing direction. The device is adapted to perform a switching operation whenever one listening party desires 5 to interrupt the talker. This reverse switching, however, must only be brought about by the party making the interrupting call and not by any transient or spurious currents. Therefore, a balancing between the voltages produced by the 50 two parties is necessary. In the blocking means it is likewise desirable that as soon as a certain direction is established by the switching means this switched-in state is to be retained until a control acting in the opposite direction is carried out.

55 All of these principal requirements demanded of an echo blocking means and feed-back blocking means can be fulfilled in an unobjectionable manner by the circuit in accordance with the invention. Reverse switching of the blocking arrange- V ment by an interrupting call'is permitted due to the fact that when transmission is in one direction then transmission in the counter direction is rendered ineffective to a limited degree. In' other words, the blocking arrangement is eifective only I for opposing voltages which exceed certain mini- 10 mum limits of differentiating values. As soon as the opposing voltages are decisively differentiated, or algebraically combined, a. reverse switching of the blocking arrangement takes place.

The control arrangement in accordance with the invention, furthermore, has the essential property of assuming a stable position in the state of rest, i. e., if none of the two control Voltages are present. This condition results in the first place from the fact that the mutual coupling between the plate circuits and grid circuits of the various tubes exists not only for alternating currents, but also for direct currents.

A tube relaxation circuit has already become known for blocking arrangements, which is infiuenced in dependence of two control voltages. This circuit differs, however, substantially from the subject mattero-f the invention in that it has but one input to which an oscillatory voltage is applied which has been produced in a special arrangement from the two control voltages by the relative balancing of said voltages. In the subject matter of the invention this additional arrangement is not required, sothat the arrange ment according to the invention is distinguished by a substantially lesser amount of means. Furthermore, in the known arrangement no possibility existed of reversing the blocking means through an intermediate call at normal volume. r '1 The basic idea of rendering a blocking arrangement ineffective to a limited degree, as explained above, is admittedly old in the art, but the apparatus that was heretofore adopted for that purpose was complicated. The present invention aims to simplify the means required.

Further details of the invention will be understood from the following description in connection with the accompanying drawing in which Figure 1 shows an illustrative circuit arrangement suitable for carrying out the invention;

Figs. 2 and 2hv are diagrams which are referred to in explaining the theory of operation of my invention;

Figs. 3 and 4 show certain. details of construc-- 55.

tion of two different embodiments of amplifiers as used in my invention; and,

Fig. 5 illustrates diagrammatically still another embodiment.

In Fig. 1 there is shown a simple circuit by way of example which embodies the principal idea of the invention. The circuit consists substantially of the two tubes R1 and R2 connected to each other in the manner of a relaxation circuit. The control action is obtained by the contacts a and b (Figs. 3 and 4) which are respectively appropriate to the two relays A and B. The windings of these relays, as shown in Fig. 1 are series connected, and it will be understood by those skilled in the art that they should be polarized so that current flow in one direction will operate relay A to close its contact while current flow in the opposite direction will operate relay B to close its contact. The inputs I and II have the two control voltages applied separately either directly or following a preceding amplification.

The biasing potentials Ugl. and Ug2 are so chosen that the tubes in the state of rest are biased close to the lower bend. Then, the arrangement falls into a position in which the one tube (for in stance, V1) has a smaller plate current i210. The plate current iao produces at the resistor R2 placed in the grid circuit of the other tube V2, an additional grid biasing potential AU; which completely blocks the tube V2 so that its plate current in returns to zero (see the plate current-grid-voltage characteristics of the two tubes shown in Fig. 2). Therefore, no current flows through the resistor R1 placed in the grid circuit of tube V1 so that also in this case no drop of the blocking potential can appear. Hence, the two windings A and B are passed only by the plate current of tube V1 in the direction shown by the arrow, and control with the contacts thereof, the speech directions such that the direction from which the voltage applied to input I is branched off, will be opened and the other direction blocked. This condition exists if no speech currents flow in the two directions of transmission.

If the one party speaks and currents arrive thereby, for instance, at the input I, the positive alternations of said currents cause a proportional control of the plate current. The plate current will be smoothed by means of the condenser C2 so that the mean value of the plate current reproduces at I the enveloping curve of the speech. The negative alternations remain without effect. This control has the result that the biasing potential of tube V2 varies in the rhythm of the enveloping speech curve. In this way speech currents of the same type appearing at the terminals II are blocked. The blocking means branched on at II is thus ineffective to a limited degree.

When the modulation of the input I ceases the circuit remains in the position characterized and plate current iao again flows through the tube V1. Now, if currents appear at II the circuit suddenly moves into the reverse position as soon as the positive alternations are higher than AUg. The relay windings are passed by current in the opposite direction and a reversal of the blocking arrangement takes place. The direction of transmission frcm which the input I is branched off will be blocked and transmission from the input II in the opposite direction will be permitted.

If in addition, i. e., during transmission in the direction assigned to the input II, speaking is done in the other direction (intermediate call) there exist always moments in which positive alternations applied to the input I are higher than the blocking potential appearing at the resistor R1. This results in a switching of the circuit so that the intermediate call can be transmitted.

In accordance with the dimensioning of the resistors R1 and R2 the possibility exists of adjusting in any desired manner the value of the voltage necessary for a switching and of adapting said value to the respective requirements of operation,

The control circuits in accordance with the invention may obviously also be used for blocking arrangements in which the blocking instead of being carried out by means of mechanical relays, is obtained through other known means such as displacement of the grid potential, or controlled rectifiers or the like. The control circuit can be placed at the transmission paths in any desired manner. The important point is hereby always that no blocking members are placed between the connection points I and II and the appertaining parties.

Figs. 3 and 4 show two examples of construction.

In Fig. 3 items V1 and V2 designate the two amplification paths of an intermediate amplifier having two wires; Items a and b designate the blocking contacts controlled by a control arrangement St. If the party connected at I speaks, the blocking voltage at II must be so high that the voltage amplified by the amplifier V1 and transmitted to the branch-off point n will be blocked. A reversing oi the control arrangement then takes place only when at the place'n a further voltage is added produced by the speech of the party connected at II.

The control arrangement may also be connected at m and 11' instead of at the points m and n. This second arrangement has the advantage that only the opposing currents need be blocked since the branches have appreciable impedance values, In general, the clampings due to the branch impedances fluctuate to a rather high degree. The value of the blocking potential should be chosen in accordance with the largest opposing current which occurs in practice. The impedances in the opposing, branch circuits may at timesbe closely matched and at other times they may be far from equal. It is, therefore, necessary to adjust the blocking potential so that, for instance when'the party I speaks, the blocking at the side II occurs with considerable voltage reserve, and when an interrupting call is made by the party II these voltages must be very high in order to cause a reversal. If, however, in case of a poor matching of the opposing currents such as would exist during transmission to the party II, so that the control potential is just enabled to reachthe per-. missible value, then low voltages from the party II are suflicient to overcome the blocking potential and to reverse the blocking.

in the drawing, the connection is made at the points m and n. In this case" each party must block its own amplified'potentials'. The working.

amplification is comparatively constant even when the line impedance is variable, so that the reserve in blocking potential can be small. The arrangement, however, operates to a lesser extent above the point of whistling but it is very reliable. Fig. 4 shows an intermediate amplifier having two wires whereby a single amplifier is suiiicient. In accordance with the position of the contacts a and b of the control circuit St an amplifier V is switched into the one or into the other direction of transmission.

. This disadvantage will be overcome if, as shown The time of continued action required in view of the passage time through the connected lines can be attained in a very simple manner by means of the circuit shown in Fig. 5. This circuit represents the left-hand part of the control circuit shown in Fig. l. The condenser-resistor combination B1, C1 placed in the grid circuit of the tube is completed by the detector Gli. If the plate current of the tube V2 flows from towards y, the condenser C1 will be charged rapidly across the passage resistance of the detector G11. Therefore, the time constant is determined substantially by the dimensions of the resistor R1 and of the condenser C1. If the plate current ceases flowing, the condenser C1 must discharge across the very high blocking resistance of the detector having the effect of a correspondingly long dying out period and, therefore, providing the desired continued action.

As compared with the hitherto known circuits, the particular advantage of the invention resides in the very accurate proportionality between the speech voltage and the blocking voltage since no dry contact rectifiers are used for rectifying the alternating potentials and, furthermore, the characteristics of the tubes can be readily made linear, for instance by means of a negative feedback.

I claim:

1. A circuit arrangement for providing a control action in dependence on two opposing voltages produced by two difierent low frequency transmission sources, said arrangement comprising a relaxation tube relay having two discharge tubes, each tube having a cathode resistor common to its own input circuit and to an output circuit appropriate to the other tube and each resistor constituting means responsive to a conductive state in one tube for blocking the other 40 tube, means for applying control potentials from each of said sources to a control grid in an appropriate one of said tubes, and relay switching means comprising two series-connected polar relay windings interconnecting the cathodes of said 45 tubes, the last said means being reversibly operable in dependence upon reversals of the conductive states of the two said tubes.

2. A switching circuit arrangement comprising two space discharge tubes each having a cathode,

0 a control grid and an anode, communication channels I and II each coupled to an input circuit including the cathode and control grid of an appropriate one of said tubes, means operative upon reception of signals on channel I only for rendering its tube conductive and for blocking the other tube, and means operative upon reception of a stronger signal on channel 11 for unblocking the tube appropriate thereto and for blocking the tube appropriate to channel I, the two said means in combination comprising a series circuit through separate windings of a polarized relay device, whereby said blocking and unblocking functions are made dependent upon the direction of current flow through said windings.

3. A circuit arrangement according to claim 2 and including switching means in circuit between the cathodes of the two tubes.

4. A circuit arrangement in accordance with claim 2. and including time delay circuits for delaying the response of said blocking and unblocking means.

5. Apparatus for blocking the transmission of signals in one direction through a communication channel while unblocking the transmission of signals in the opposite direction through sad channel, said apparatus comprising polar relay means having circuit closing contacts in circuit with a uni-directional branch of said channel, and other circuit closing contacts alternatively operable upon reversal of current through said polar relay means, said other contacts being in circuit with a second uni-directional branch of said channel, the two said branches being arranged for mutual- 1y exclusive transmission of signals in opposite directions, a pair of discharge tubes each having a cathode, an anode and a control grid, a grid bias resistor for each tube forming part of the anode circuit of the other tube, separate anode potential sources in each anode circuit, each said resistor being disposed in a cathode-grid circuit appropriate to a particular tube, and a trans-.

former secondary winding in circuit between each grid and its bias resistor, each said secondary windin being responsive to signals derived from an appropriate direction through said communication channel, for rendering its associated tube conductive, said apparatus being further characterized in that said polar relay means comprises two series-connected operating windings connected between two cathode leads, each of which forms a common input-outputportion of its respective tube circuit.

ERWIN HtiLzLER. 

